CMOS circuits are known for their low power dissipation. Demands for higher performance have resulted, however, in decreasing cycle times and increasing clock frequencies, thereby increasing demand for CMOS driver circuits consuming minimal power.
With the higher operating frequencies demanded by state-of-the-art CMOS microprocessors, a little-known effect of CMOS circuits now accounts for a significant portion of the total overall power dissipation. In some recent designs, the clock distribution circuitry has consumed almost 40% of the total microprocessor chip power.
The effect occurs during a signal transition as the input and output signals of any CMOS static circuit change state. During the transition, a current path is created that runs directly from the V.sub.cc positive power supply conductor to ground (GND). Referring to FIG. 1a, this current path is labeled Isc, and current flowing through this path is referred to as "pass-through current." A sample waveform is shown in FIG. 1b. As the input signal changes to produce an output signal change, there exists a period of time when both the upper P-type device(s) Q.sub.1 and the lower N-type device(s) Q.sub.2 are turned on, thus creating the path from V.sub.cc to GND. The resultant current, Isc, is made larger as the input waveform transition time is increased up to a maximum peak value, where further increase in rise time does not produce any further increase in Isc peak value. Nevertheless, the Isc pulse width continues to widen causing the power=V.sub.cc * Isc(AVG) to increase further.
Although a simple CMOS inverter is depicted in FIG. 1a, this problem can be found in any CMOS static combinatorial circuit configuration such as a NAND or NOR logical function. There exists a need in the art, therefore, for a CMOS driver circuit that minimizes or eliminates the pass-through current characteristic seen in earlier designs, such as that shown in FIG. 1a. Reducing or eliminating the Isc pass-through current would reduce the power consumed by the circuit, and would also reduce the heat and noise generated by the circuit.